On these cars you can not get your speedo recalibrated. This site will tell you the difference in speed
http://www.miata.net/garage/tirecalc.html
On my GT when I went from factory 205/50-17 to 205/40-17 my speedo is now 6.4% too fast. At 60 mph I am actually doing 56.1 mph. I just compensate with a 4 mph difference. You can adjust going to a bigger size tire by going to a lower profile tire if you want, in other words if I went to a 205/40-18 I would only be reading 2.4% too fast and going 58.5 mph instead of 60 mph. You can play with the numbers and see what you want.